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United States Patent |
5,085,265
|
Yamamoto
,   et al.
|
February 4, 1992
|
Method for continuous casting of molten steel and apparatus therefor
Abstract
An apparatus for continuous casting of molten steel comprises a continuous
casting mold; an electromagnetic stirring coil, which rotates and
fluidizes molten steel inside the mold and which is installed outside the
mold; and a screen of ferromagnetic substance positioned between the mold
and the electromagnetic stirring coil at a height including a level of
meniscus. A method for continuous casting of molten steel comprises the
steps of pouring molten steel into a continuous casting mold; applying an
electromagnetic force to the molten steel in the mold by means of an
electromagnetic coil installed outside the continuous casting mold; and
shielding said electromagnetic force by means of a screen of ferromagnetic
substance installed between the mold and electromagnetic coil at a height
including a level of meniscus.
Inventors:
|
Yamamoto; Hironori (Kawasaki, JP);
Matsumura; Chitoshi (Kawasaki, JP);
Mori; Kentaro (Kawasaki, JP)
|
Assignee:
|
NKK Corporation (Tokyo, JP)
|
Appl. No.:
|
672373 |
Filed:
|
March 20, 1991 |
Foreign Application Priority Data
Current U.S. Class: |
164/468; 164/504 |
Intern'l Class: |
B22D 027/02 |
Field of Search: |
164/502,503,504,466,467,468
|
References Cited
U.S. Patent Documents
4273180 | Jun., 1981 | Tertishnikov et al. | 164/503.
|
4523627 | Jun., 1985 | Cans et al. | 164/503.
|
4530404 | Jul., 1985 | Vives | 164/467.
|
Foreign Patent Documents |
1-215439 | Aug., 1989 | JP | 164/503.
|
Primary Examiner: Lin; Kuang Y.
Attorney, Agent or Firm: Frishauf, Holtz, Goodman & Woodward
Claims
What is claimed is:
1. An apparatus for continuous casting of molten steel, comprising:
a continuous casting mold;
an electromagnetic stirring coil for generating a shifting magnetic field
shifting in a horizontal plane, which rotates and flows molten steel
inside said mold and which is installed outside said mold; and
a screen of ferromagnetic substance positioned between said mold and the
electromagnetic stirring coil at a height including a level of meniscus.
2. The apparatus of claim 1, wherein said screen of ferromagnetic substance
constitutes an upper portion of an inner vessel positioned between the
continuous casting mold and the electromagnetic stirring coil.
3. The apparatus of claim 1, wherein said screen of ferromagnetic substance
is installed in an upper portion of an inner vessel positioned between the
continuous casting mold and the electromagnetic stirring coil and outside
the inner vessel.
4. The apparatus of claim 1, wherein said screen of ferromagnetic substance
is installed in the range of from the top end of the continuous casting
mold to a position of 200 mm downward from the top end of the continuous
casting mold.
5. The apparatus of claim 1, wherein said ferromagnetic substance is
selected from a group consisting of pure iron, common steel, ferrite,
cobalt and nickel.
6. The apparatus of claim 1, wherein said screen of ferromagnetic substance
has a thickness of from 10 to 25 mm.
7. The apparatus of claim 1, wherein
said screen of ferromagnetic substance constitutes an upper portion of an
inner vessel positioned between the contiuous casting mold and the
electromagnetic stirring coil;
said screen of ferromagnetic substance is installed in the range of from
the top end of the continuous casting mold to a position of 200 mm
downward from the top end of the continuous casting mold; and
said ferromagnetic substance is common steel.
8. A method for continuous casting of molten steel, comprising the steps
of:
pouring molten steel into a contiuous casting mold;
applying an electromagnetic force to the molten steel in said mold by means
of a shifting magnetic field generated by an electromagnetic coil
installed outside the continuous casting mold wherein said magnetic field
shifting in a horizontal plane; and
shielding said electromagnetic force by means of a screen of ferromagnetic
substance installed between said mold and said electromagnetic coil at a
height including a level of meniscus.
9. The method of claim 8, wherein said screen of ferromagnetic substance
constitutes an upper portion of an inner vessel positioned between the
continuous casting mold and the electromagnetic stirring coil.
10. The method of claim 8, wherein said screen of ferromagnetic substance
is installed in an upper portion of an inner vessel positioned between the
continuous casting mold and the electromagnetic stirring coil and outside
the continuous casting mold.
11. The method of claim 8, wherein said screen of ferromagnetic substance
is installed in the range of from the top end of the continuous casting
mold to a position of 200 mm downward from the top end of the continuous
casting mold.
12. The method of claim 8, wherein said screen of ferromagnetic substance
is installed in the range of from a position of 100 mm upward from
meniscus to a position of 100 mm downward from the meniscus.
13. The method of claim 8, wherein said ferromagnetic substance is selected
from a group consisting of pure iron, common steel, ferrite and cobalt.
14. The method of claim 8, wherein said screen of ferromagnetic substance
has a thickness of from 10 to 25 mm.
15. The method of claim 8, wherein said electromagnetic force has magnetic
flux density of from 200 to 800 Gauss.
16. The method of claim 8, wherein
said screen of ferromagnetic substance constitutes an upper portion of an
inner vessel positioned between the continuous casting mold and the
electromagnetic stirring coil;
said screen of ferromagnetic substance is installed in the range of from
the top end of the continuous casting mold to a position of 200 mm
downward from the top end of the continuous casting mold;
said ferromagnetic substance is common steel; and
said electromagnetic force has magnetic flux density of from 200 to 800
Gauss.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method for continuous casting of molten
steel and apparatus therefor, and more particularly to a method for
continuously casting molten steel by electromagnetically stirring molten
steel and apparatus therefor.
2. Description of the Related Arts
As a method for minimizing center segregations of solidification structure
by increasing fine equi-axed crystals, there are pointed out a
low-temperature casting method and an electromagnatic stirring method. In
the low-temperature casting method, inhomogeneous nuclei are easily
produced by making superheating of the molten steel as small as possible
during casting of liquid metal. This method wherein fine equi-axed
crystals can be obtained is known as the simplest method for improving the
solidification structure.
In the elecromagnetic stirring method, the equi-axed crystal structure is
obtained by dividing dendrite arms by forcedly flowing molten steel
adjacent to a solidification interface. As the electromagnetic stirring
method, there are pointed out a linear motor type, rotary type and
magnetostatic field type electromagnetic stirring methods. In the linear
motor type and rotary type electromagnetic stirring methods, a shifting
magnetic field is applied to molten steel, and the molten steel is
forcedly flowed by an interaction of an eddy current generated in the
molten steel with the applied magnetic field. In the magnetostatic
electric field type electromagnetic stirring method, Lorentz's force is
obtained by constantly feeding electric current to molten steel, to which
a static magnetic field is applied.
FIG. 7 is an explanatory view showing a situation wherein molten steel
adjacent to a meniscus inside a continuous casting mold is stirred by the
rotary type electromagnetic stirring apparatus along the inner
circumferential surfaces of the mold.
An electromagnetic stirring coil 22 surrounding the continuous casting mold
21 is positioned at a level of a height containing the meniscus of the
molten steel outside the continuous casting mold 21. The molten steel is
stirred by generating a rotating magnetic field inside the mold by means
of the electromagnetic coil 22. Dendrite arms generated along the inner
circumferential surfaces of the mold 21 are divided by this stirring
whereby an equi-axed crystal structure is obtained.
An electromagnetic stirring force has to be increased to increase the ratio
of equi-axed crystals. Since molten steel adjacent to the inner
circumference of the mold is raised by a centrifugal force as shown in
FIG. 7 when the electromagnetic stirring force is increased, the thickness
of a powder pool 24 of lubricating powder on the molten steel 23 inside
the continuous casting mold 21 becomes small. Unmelted powder is entrapped
into the molten steel whereby slag spots are generated. As the result that
powder flows non-uniformly into between the mold 21 and a solidified shell
since air is included into the powder, the powder pool being flowed, the
rate of solidification of the molten steel becomes small in parts. In
consequence, longitudinal cracks are generated on the surface of a billet.
Further, when a flow of molten steel is generated in front of the
solidified shell by the use of an electromagnetic stirring apparatus, a
negative segregation zone is generated since a concentrated molten steel
among the dendrites is washed.
Japanese Patent Publication Laid Open No. 70361/89 discloses a method
wherein an electromagnetic coil is arranged in an outer circumference of a
continuous casting mold and a round electrically conductive ring is
arranged adjacent to a meniscus of the molten metal to apply
perpendicularly and upwardly a magnetic field to molten metal. However,
this method does not relate to the rotating elecromagnetic stirring
method.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a rotating
electromagnetic stirring method wherein the ratio of equi-axed crystals
can be raised without generating any slag spot and longitudinal crack in
steel and an apparatus therefore.
To attain the aforementioned object, the present invention provides an
apparatus for continuous casting of molten steel, comprising:
a continuous casting mold;
an electromagnetic stirring coil, which rotates and flows molten steel
inside said mold and which is installed outside said mold; and
a screen of ferromagnetic substance positioned between said mold and the
electromagnetic stirring coil at a height including a level of meniscus.
The present invention further provides a method for continuous casting of
molten steel, comprising the steps of:
pouring molten steel into a continuous casting mold;
applying an electromagnetic force to the molten steel in said mold by means
of an electromagnetic coil installed outside the continuous casting mold;
and
shielding said electromagnetic force by means of a screen of ferromagnetic
substance arranged between said mold and electromagnetic coil at a height
including a level of meniscus.
The above objects and other objects and advantages of the present invention
will become apparent from the following detailed description, taken in
conjunction with the appended drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a vertical sectional view illustrating an apparatus for
continuous casting of molten steel of the present invention;
FIG. 2 is a vertical sectional view illustrating another apparatus for
continuous casting of molten steel of the present invention;
FIG. 3 is a graphical representation designating the relationship between
the distance of from the top end of a mold to the lower side thereof and
themagnetic flux density according to the present invention;
FIG. 4 (A) is a graphical representation designating the relationship
between the electric current of the electromagnetic stirring coil and the
ratio of equi-axed crystals according to the present invention;
FIG. 4 (B) is a graphical representation designating the relationship
between the electric current of the electromagnetic stirring coil and the
index of the longitudinal cracks according to the present invention;
FIG. 4 (C) is a graphical represenation designating the relationship
between the electric current of the electromagnetic stirring coil and the
index of slag spots according to the present invention;
FIG. 5 is a graphical representation designating the distribution of
concentration of carbon in the radial direction of a billet according to
the present invention;
FIG. 6 is a graphical reprsentation designating the relationship between
the electric current of the electromagnetic stirring coil and the maximum
degree of negative segregation according to the present invention;
FIG. 7 is a schematic illustration showing a prior art rotating
electromagnetic stirring apparatus; and
FIG. 8 is a graphical representation designating the relationship between
the thickness of a shield and the decay ratio of the magnetic flux
density;
FIG. 9 is a graphical representation showing the relationship between the
distance from the meniscus and the stirring flow velocity according to the
present invention; and
FIG. 10 (A) to (C) are schematic illustration showing a distribution of
magnetic flux of coil for rotating and flowing molten steel according to
the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
The apparatus for continuous casting of molten steel of the present
invention comprises a continuous casting mold, an electromagnetic stirring
coil and a screen of ferromagnetic substance. The electromagnetic stirring
coil is installed outside the mold to cause molten steel inside the mold
to rotate and to be flowed. The screen is positioned between the mold and
the electromagnetic stirring coil at a height including a level of
meniscus.
The reason for the arrangement of the aforementioned screen is as follows:
When a great stirring force is imparted to enhance the ratio of equi-axed
crystals by the rotating electromagnetic stirring apparatus without any
center segregation, the thickness of the powder pool on the molten steel
is decreased since the molten steel adjacent to the inner circumferential
surface of the mold is raised by a centrifugal force. Since the thickness
of the pool is decreased, slag spots and longitudinal cracks are generated
in a billet. Accordingly, it is sufficient to weaken the stirring force of
the molten steel adjacent to the miniscus so that the thickness of the
powder pool cannot be decreased. The periphery of the powder pool is
prevented from swelling. It is sufficient to absorb a magnetic flux acting
on the periphery of the meniscus. In the apparatus for continuous casting
of molten steel, a screen of ferromagnetic substance such as pure iron,
steel or the like is installed between the electromagnetic stirring coil
and the continuous casting mold around the mold at a height including a
level of meniscus. The magnetic flux passing through a portion of the
meniscus is shielded by the screen.
FIG. 8 is a graphical representation designating the relationship between
the thickness of materials shielding the molten steel from the magnetic
flux when the frequency of electrical current caused to pass through the
electromagnetic stirring coil is 50 Hz and the decay ratio of the magnetic
flux density. In the drawing, A denotes the case of air, B the case of
stainless steel of austenite of 1000.degree. C., and C the case of iron of
30.degree. C. When the ferromagnetic substance such as pure iron, steel or
the like is used, the magnetic flux does not pass substantially through
the materials shielding the molten steel when the molten steel is shielded
by a plate of from 10 to 25 mm in thickness. As for the frequency of
electric current caused to pass through the electromagnetic stirring coil,
a low-frequency power source of from 2 to 20 Hz is desired to be used to
prevent the magnetic flux density from damping in a mold of copper plate.
The degree of absorption of the magnetic flux by the ferromagnetic
substance is equal to that in FIG. 8.
The apparatus for continuous casting of molten steel of the present
invention will now be described with specific reference to FIG. 1.
The apparatus for continuous casting of molten steel is composed of an
outer vessel 2 positioned most outside, an inner vessel 3 inserted into
the outer vessel 2 and a tubular mold 4 which is inserted into the inner
vessel 3 and forms a solidification shell from molten steel by contacting
the molten steel. A cooling water path 5 is formed between the inner
vessel 3 and the tubular mold 4, which is constantly cooled by cooling
water. A ring-shaped concave portion 6 is positioned in a portion where
the outer vessel 2 contacts the inner vessel 3 in the continuous casting
mold. An electromagnetic stirring coil 7 is installed in the concave
portion 6. The inner vessel is composed of an upper portion and a lower
portion. The upper portion of the inner vessel 3 is a screen 8 made of
common steel of ferromagnetic substance such as steel SS 41 or the like.
The common steel in the upper portion of the inner vessel is connected to
stainless steel in the lower portion of the inner vessel by welding. In
this Preferred Embodiment, the above-mentioned screen of ferromagnetic
substance is positioned in the range of from the top end of the mold to a
position of 200 mm from the top end of the mold. That is, the screen is
positioned in the range which ranges 100 mm upwardly and downwardly with
the height of the meniscus as the center.
If there is a gap large enough to put the screen into between the inner
vessel 3 and the electromagnetic stirring coil 7, a screen of common steel
is wound around the outer surface of the inner vessel of stainless in the
form of a headband as shown in FIG. 2 and can be fixed to the inner vessel
3 by bolts or the like. When the screen 8 is put between the mold 4 and
the coil 7, elecromagnetic energy absorbed by the screen 8 converts to
heat. However, since the screen 8 together with the mold 4, the inner
vessel 3 and the coil 7 are cooled by water, the screen cannot be
overheated. Pure iron, common steel, ferrite, cobalt, nickel or the like
is used for the screen.
A three-phase two-poles electromagnetic stirring coil 7 of 561 mm in
outside diameter, 350 mm in inside diameter and 400 mm in length having a
maximum coil capacity of 1000 Gauss was used. In this example, a
three-phase two-poles coil 7 was used.
A two-phase two poles of three phase four-poles electromagnetic coil can be
used.
A distribution of magnetic flux in coils flowing rotationally molten steel
is shown in FIG. 10 (A) to (C). FIG. 10 (A) shows a case of using a
three-phase four-poles electromagnetic coil, FIG. 10 (B) a case of using a
three-phase two poles electromagnetic coil and FIG. 10 (C) a case of using
a two-phase two-poles electromagnetic coil.
Subsequently, a method for producing steel by the use of the continuous
casting apparatus of the present invention will now be described.
FIG. 3 is a graphical representation designating the relationship between
the distance of from the top end of the mold to the lower side of the mold
and the magnetic flux density according to the present invention. Electric
current of 100 A and 200 A was passed through the electromagnetic coil 7,
and it was studied how the magnetic flux density was changed in the range
of from the top end of the mold to the lower side. FIG. 3 shows a case
with screen where the magnetic flux density is shown by .quadrature. when
the electric current was of 100 A and by when the electric current was
200 A. FIG. 3 also shows a case without screen where the magnetic flux
density is shown by .largecircle. when the electric current was 100 A and
by when the electric current was 200 A. When the magnetic flux was not
shielded by the screen of ferromagnetic substance, the magnetic flux
density became large at a position of 100 mm downward from the top end of
the mold, that is, from a position adjacent to the meniscus of molten
steel 10 which powder 9 contacted. Conversely, when the magnetic flux was
shielded by the screen, the magnetic flux density was low in the range of
from the top end of the mold to a position of 200 mm from the top end of
the mold, and large enough to obtain a stirring force at a position
downward from the position of 200 mm downward from the top end of the
mold. A flow velocity of the molten steel in the portion of the meniscus
was 20 cm/sec., which was a flow velocity enabling the powder to uniformly
flow into the molten steel. The flow velocity of the molten steel was 80
cm/sec. at a depth of 500 mm from the top end of the mold. A sufficient
stirring force could be obtained by this flow velocity.
The maximum magnetic flux density applied to the molten steel is desired to
be from 200 to 800 Gauss.
FIG. 4 (A) to (C) are graphical representations desiganting the
relationship between the inner property and surface quality of a billet
when the billet having a chemical composition corresponding to that of
carbon steel S 45 C for mechanical structure and a size of 170 mm in
diameter was produced at a casting speed of 1.8 m/min. The carbon steel
contained 0.45 wt.% carbon and 0.8 wt.% manganese.
FIG. 4 (A) is a graphical representation designating the relationship
between the electrical current of the electromagnetic stirring coil and
the ratio of area of equi-axed crystals. The ratio of area of equi-axed
crystals is obtained by revealing a macro-structure of steel by applying a
hydrochloric acid treatment to a section of a billet, measuring a
thickness of accumulation of the equi-axed crystals and finding the ratio
of area of the equi-axed crystals to the section of the slab. As shown in
Table 1, symbols in FIG. 4 are distinguished by superheating degrees
.DELTA.T ( .degree. C.) from a liquidus line of steel and cases with
screen and without screen.
TABLE 1
______________________________________
.DELTA.T (.degree.C.)
10.about.20
20.about.30
30.about.40
40.about.
______________________________________
with screen .quadrature.
.DELTA. .circle.
without screen
______________________________________
Generally, to increase an inner cleanliness of a billet produced by
casting, it is good that .DELTA.T is about 20.degree. C. or more.
Conversely, it is said that when .DELTA.T is increased, the ratio of area
of equi-axed crystals is lowered. Since the ratio of area of equi-axed
crystals is not decreased in the method for continuous casting of molten
steel even when .DELTA.T is increased, steel whose ratio of area of
equi-axed crystals is large and whose cleanliness is high can be obtained.
FIG. 4 (B) is a graphical representation designating the relationship
between the electric current of the electromagnetic stirring coil and the
index of the longitudinal cracks. The index of the longitudinal cracks is
a value (mm/m) obtained by applying a slight hydrochloric acid teatment to
the surface of a billet, finding a total amount of lengths of the
longitudinal cracks revealed, dividing the total amount of lengths of the
longitudinal cracks by the length of the billet.
In the drawing, symbol .largecircle. denotes a case with screen and symbol
a case without screen.
FIG. 4 (C) is a graphic representation desiganting the relationship between
the electric current of the elecromagnetic stirring coil and the index of
the slag spots. The index of the slag spots is a value (number/m) obtained
by cutting the outer surface of a billet by 1 mm, finding a total number
of inclusions of unmelted powder or molten powder, which appear on cut
surface of the billet and dividing the total number of inclusions by the
length of the billet. In the drawing, symbol .largecircle. denotes a case
with screen and symbol a case without screen.
As clearly seen from FIG. 4 (B) and FIG. 4 (C), both the index of the slag
spots (number/m ) and the longitudinal cracks (mm/m ) do not become worse
even when the value of electric current, namely, the stirring force is
increased. That is, it is shown that the inner property of the billet can
be enhanced, keeping the ratio of are of equi-axed crystals as shown in
FIG. 4 (A) at the same level as that in the prior art electromagnetic
stirring.
FIG. 5 is a graphical representation showing the distribution of carbon in
the radial direction of the billet when the billet was produced by
electromagnetically stirring molten steel with coil current of 300 ampere
(A)by using the continuous casting apparatus of the present invention. The
section of the billet was 170 mm. The casting speed was 1.5 m/min. In the
drawing, symbol .largecircle. denotes a case with screen and symbol a
case without screen. When molten steel before solidification is generally
flowed by using an electromagnetic stirring apparatus, a negative
segregation zone is generated because concentrated molten steel before a
solid phase is taken away. When this negative segregation zone is
generated, the size of the billet is not stable in the case of plastic
working of the billet due to the change of properties of the billet in the
radial direction thereof in the case of the occurrence of the negative
segregation zone. Since the hardness of steel is lowered in the negative
segregation zone, for example, the size of the steel is not stable after
the working of the steel. As shown by a white circle ( .largecircle. ) in
FIG. 5, when the continuous casting apparatus using the screen of the
present invention is used, this negative segregation is decreased whereby
a billet having a highly homogeneous property under the surface layer of
the billet.
FIG. 6 is a graphical representation designating the relationship between
the maximum value of the negative segregation and the effect of the
present invention. Symbols in the drawing are distinguished by the casting
speed (m/min) and cases with screen and without screen and shown in Table
2.
TABLE 2
______________________________________
Vc (m/min)
1.2 1.5 1.8 2.0
______________________________________
With screen .quadrature.
.DELTA. .circle.
--
Without screen
______________________________________
As seen from symbols .quadrature., .DELTA. and .largecircle., when the
screen is used, the maximum degree of the negative segregation is 0.92 or
more, which is practically unharmed. Judging by combining FIG. 6 with FIG.
3 (A) to (C), it is understood that the billet is suerior in its inner
property to the billet in the case without screen, and the billet having a
high homogeneity under the surface layer of he billet is produced.
FIG. 9 is a graphical representation showing the relationship between the
distance from meniscus and the stirring velocity. Symbols in the drawing
are distinguished by the casting speed (m/min. ) and the case with screen
and the case without screen. The symbols are the same as those shown in
Table 2. The stirring flow velocity is represented by the following
equation:
##EQU1##
where U: stirring velocity
V: solidification rate
ke: degree of negative segregation
ko: equilibrium distribution coefficient
L: distance from meniscus
k: solidification coefficient
Vc: casting speed
In the drawing, A denotes an upper limit of the flow velocity of molten
steel and B a lower limit of the flow velocity of the molten steel. The
flow velocity of the molten steel in the portion of miniscus is desired to
be of from 25 to 50 cm/sec. because slag spots are liable to occur when
the flow velocity of the molten steel exceeds 50 cm/sec. and blow holes
are liable to occur when the flow velocity of the molten steel is below 25
cm/sec. The stirring velocity is desired to be 70 cm/sec. or less just
under the meniscus. When the stirring velocity exceeds 70 cm/sec., an
amount of molten steel raised by the centrifugal force adjacent to the
inner circumference of the mold is increased and the thickness of powder
pool on the molten steel is decreased. Then, unmelted powder is included
into the molten steel, which generates slag spots. The stirring velocity
of the molten steel is desired to be of from 30 to 45 cm/sec. at a
position of 0.2 m downward from meniscus.
White band is not generated in this range.
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